The present invention relates to fluorescent substances, especially suitably used for vacuum ultraviolet radiation excited light-emitting devices such as plasma display panels (hereinafter referred to as xe2x80x9cPDPsxe2x80x9d) and rare gas lamps.
Fluorescent substances that emit light when excited with vacuum ultraviolet radiation or the like include, for example, BaMgAl10O17:Eu comprising Ba, Mg, Al, O and an activator (Eu) as a blue fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices, and Zn2SiO4:Mn comprising Zn, Si, O and an activator (Mn) as a green fluorescent substance. As well, (Y,Gd)BO3:Eu comprising Y, Gd, B, O and an activator (Eu), for example, has been known as a red fluorescent substance. Such fluorescent substances are also used in rare gas lamps. These prior art fluorescent substances for vacuum ultraviolet radiation excited light-emitting devices, however, are desired to exhibit further enhanced luminance.
In vacuum ultraviolet radiation excited light-emitting devices such as PDPs and rare gas lamps, causing discharge in a rare gas generates plasma and then, vacuum ultraviolet radiation is generated. The prior art fluorescent substances involve a problem that the luminance thereof decreases as they are exposed to plasma. Thus, there is a desire for a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices of which luminance does not much decrease due to exposure to plasma.
U.S. Pat. No. 5,839,718 discloses a silicate fluorescent substance comprising a compound represented by the general formula: m(Sr1-aM1a)Oxc2x7n(Mg1-bM2b)Oxc2x72(Si1-cGec)O2:EuxLny, wherein M1 is Ba, M2 is at least one element selected from the group consisting of Be, Zn and Cd, Ln is at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, B, Al, Ga, In, Tl, Sb, Bi, As, P, Sn, Pb, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Cr and Mn. Specific examples of such silicate fluorescent substances disclosed include Sr1.995MgSi2O7:Eu0.005, Dy0.025, Cl0.025, and Sr0.445Ba1.55MgSi2O7:Eu0.005, Dy0.025, Cl0.025. These fluorescent substances are for use as light accumulators for display in dark places or for similar purposes. xe2x80x9cExtended abstracts of the sixth international conference on the science and technology of display phosphorsxe2x80x9d, pages 21-24, discloses CaMgSi2O6:Eu as a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices.
Such fluorescent substances comprising silicate still have the problem that their luminance is insufficient and decreases due to exposure to plasma.
It is an object of the present invention to provide a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices which emits light with a higher luminance and a small decrease in luminance after the exposure to plasma.
Under such circumstances, the inventors of the present invention have made an intensive and continuous study in pursuit of resolving the foregoing problems and, as a result, have found that, among silicate or germanate fluorescent substances for vacuum ultraviolet radiation excited light-emitting devices, a fluorescent substance having a specific composition comprising: a compound represented by the general formula, mM1Oxc2x7nM2Oxc2x72M3O2, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and m and n satisfy 0.5xe2x89xa6mxe2x89xa63.5 and 0.5xe2x89xa6nxe2x89xa62.5, respectively, provided that when m=n=1, M1 is either at least two metals selected from the group consisting of Ca, Sr and Ba, or one of Sr and Ba; and either one of Eu or Mn as an activator, emits light with a higher luminance and a smaller decrease in luminance after the exposure to plasma. Thus, the present invention has been completed.
Accordingly, the present invention provides a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices, comprising: a compound represented by the general formula, mM1Oxc2x7nM2Oxc2x72M3O2, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and m and n satisfy 0.5xe2x89xa6mxe2x89xa63.5 and 0.5xe2x89xa6nxe2x89xa62.5, respectively, provided that when m=n=1, M1 is either at least two metals selected from the group consisting of Ca, Sr and Ba, or one of Sr and Ba; and either one of Eu or Mn as an activator. The present invention also provides a vacuum ultraviolet radiation excited light-emitting device comprising the fluorescent substance defined above.
The fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices according to the present invention is characterized by comprising: a base crystal formed of a compound represented by the general formula: mM1Oxc2x7nM2Oxc2x72M3O2, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and m and n satisfy 0.5xe2x89xa6mxe2x89xa63.5 and 0.5xe2x89xa6nxe2x89xa62.5, respectively, provided that when m=n=1, M1 is either at least two metals selected from the group consisting of Ca, Sr and Ba, or one of Sr and Ba; and either one of Eu or Mn as an activator.
When m=n=1 in the above-noted general formula, M1 is selected from the group consisting of Sr, Ba, the combination of Sr, Ba and Ca, combination of Sr and Ca, combination of Ba and Sr, and combination of Ba and Ca.
Preferably, the fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices according to the present invention has a crystal structure as same as diopside, akermanite, or merwinite.
Among fluorescent substances having the same crystal structure as diopside, one comprising a compound of the above-noted general formula where m=n=1 is preferable. A more preferable one as a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices is of a composition in which M1 is partially replaced with the activators Eu, or M2 is partially replaced with the activators Mn, and which is represented by the general formula: (M11-aEua)M2M32O6 or M1(M21-bMnb)M32O6, wherein M1 is either at least two metals selected from the group consisting of Ca, Sr and Ba, or one of Sr and a, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and a and b satisfy 0xe2x89xa6axe2x89xa60.5 and 0xe2x89xa6bxe2x89xa60.5, respectively.
A still preferred one of fluorescent substances having the same crystal structure as diopside and comprising a compound of the above-noted general formula where m=n=1 has a composition in which M1 is Ca and Sr, M2 and M3 are Mg and Si, respectively, and Ca is partially replaced with the activator Eu and which is represented by the general formula: Ca1-cSrcEudMgSi2O6, wherein c and d satisfy 0 less than cxe2x89xa60.1 and 0 less than dxe2x89xa60.1, respectively.
Among fluorescent substances having the same crystal structure as akermanite, one comprising a compound of the above-noted general formula where m=2 and n=1 is preferable. A more preferable one as a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices has a composition in which M1 is partially replaced with the activators Eu, or M2 is partially replaced with the activators Mn, respectively, and which is represented by the general formula: (M11-eEue)2M2M32O7 or M12(M21-fMnf)M32O7, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and e and f satisfy 0xe2x89xa6e greater than 0.5 and 0xe2x89xa6fxe2x89xa60.5, respectively. Still more preferable is a fluorescent substance having a composition in which f=0, and M3 is Si and which is represented by the general formula: (M11-eEue)2M2 Si2O7, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, and e satisfies 0.001xe2x89xa6exe2x89xa60.1.
Among fluorescent substances having the same crystal structure as akermanite, one comprising a compound of the above-noted general formula where m=1 and n=2 is preferable. A more preferable one as a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices has a composition in which M1 is partially replaced with the activators Eu, or M2 is partially replaced with the activators Mn, respectively, and which is represented by the general formula: (M11-hEuh)M22M32O7 or M1(M21-iMni)2M32O7, wherein M is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and h and i satisfy 0xe2x89xa6hxe2x89xa60.5 and 0 xe2x89xa6ixe2x89xa60.5, respectively.
Among fluorescent substances having the same crystal structure as merwinite, one comprising a compound of the above-noted general formula where m=3 and n=1 is preferable. A more preferable one as a fluorescent substance for vacuum ultraviolet radiation excited light-emitting devices has a composition in which M1 is partially replaced with the activators Eu, or M2 is partially replaced with the activators Mn, respectively, and which is represented by the general formula: (M11-jEuj)3M2M32O8 or M13(M21-kMnk)M32O8, wherein M1 is at least one metal selected from the group consisting of Ca, Sr and Ba, M2 is at least one metal selected from the group consisting of Mg and Zn, M3 is at least one metal selected from the group consisting of Si and Ge, and j and k satisfy 0xe2x89xa6jxe2x89xa60.5 and 0 xe2x89xa6kxe2x89xa60.5, respectively.
Among fluorescent substances having the same crystal structure as diopside, akermanite or merwinite, those having the same crystal structure as diopside and those having the same crystal structure as merwinite are preferable. Fluorescent substances having the same structure as diopside are particularly preferable.
Raw materials usable as calcium, strontium and barium sources in the preparation of the fluorescent substance of the present invention may be either those which can be decomposed into oxides at high temperatures such as hydroxides, carbonates, nitrates, halides, oxalates or the like of calcium, strontium and barium having high purity (99% or more), or oxides of calcium, strontium and barium having high purity (99.9% or more).
Raw materials usable as magnesium and zinc sources may be either those which can be decomposed into oxides at high temperatures such as hydroxides, carbonates, nitrates, halides, oxalates or the like of magnesium and zinc having high purity (99% or more), or oxides of magnesium and zinc having high purity (99% or more).
Raw materials usable as silicon and germanium sources may be either those which can be decomposed into oxides at high temperatures such as hydroxides, carbonates, nitrates, halides, oxalates or the like of silicon and germanium having high purity (99% or more), or oxides of silicon and germanium having high purity (99% or more).
Raw materials containing europium or manganese that will serve as an activator may be either those which can be decomposed into oxides at high temperatures such as hydroxides, carbonates, nitrates, halides, oxalates or the like of europium and manganese having high purity (99% or more), or oxides of europium and manganese having high purity (99% or more).
There is no particular limitation to the method of preparing the fluorescent substance of the present invention. The fluorescent substance may be prepared, for example, by mixing the aforementioned raw materials and calcining the mixture.
For instance, the fluorescent substance having the composition represented by the formula: (M11-aEua)M2M32O6, which is one of the preferable compositions described above, can be prepared by weighing and blending the foregoing raw materials into a predetermined composition, mixing them and calcining the mixture. In mixing such raw materials, it is possible to use a ball mill, V-Type mixer, stirring device or the like.
After the mixing, the mixture is calcined at a temperature, for example, ranging between about 1000xc2x0 C. and about 1500xc2x0 C. for about 1 to about 100 hours, to give the fluorescent substance of the present invention. In the case where those raw materials which can be decomposed into oxides at high temperatures such as hydroxides, carbonates, nitrates, halides, oxalates or the like of the necessary elements are used, it is possible to conduct pre-calcination of the mixture at a temperature, for example, ranging between about 600xc2x0 C. and about 900xc2x0 C. prior to the main-calcination.
There is no particular limitation to the calcining atmosphere, but calcining in a reducing atmosphere comprising, for example, nitrogen, argon or the like and hydrogen in an amount of 10% by volume is preferable. The pre-calcination may be performed either in atmospheric air or in a reducing atmosphere. To promote the calcining reaction, an appropriate amount of a flux may be added.
Further, the fluorescent substance obtained by the above-described method may be pulverized using, for example, a ball mill, jet mill or the like. Further, the substance may be washed and classified. To obtain a fluorescent substance of higher crystallinity, re-calcination may be performed.
The fluorescent substance of the present invention thus obtained is capable of emitting light with a high luminance when excited with vacuum ultraviolet radiation and exhibits only a small decrease in luminance due to exposure to plasma. In the manufacture of a PDP, rare gas lamp or the like, a process typically includes dispersing the fluorescent substance in a solvent, adding a binder to the dispersion liquid, applying the dispersion liquid onto alight-emitting section, and heating the dispersion liquid thus applied to about 500xc2x0 C. to remove the binder. The fluorescent substance of the present invention, even if subjected to such a process, exhibits only a small decrease in luminance due to the heat treatment. Accordingly, use of the fluorescent substance of the present invention in a vacuum ultraviolet radiation excited light-emitting device such as PDP or rare gas lamp makes it possible to realize a PDP or rare gas lamp exhibiting a high luminance and a prolonged service life. Hence, the fluorescent substance of the present invention is suitable for vacuum ultraviolet radiation excited light-emitting devices.
The fluorescent substance of the present invention can also be excited with ultraviolet radiation out of the vacuum ultraviolet region, X-rays, electron beam or the like and hence may be employed in devices using ultraviolet radiation out of the vacuum ultraviolet region, X-rays or electron beam as an excitation source.